1. Field of the Invention
The present invention relates to a high voltage detection circuit and a bell signal detection circuit and a non-ringing reception detection circuit for a telephone line terminal employing the high voltage detection circuit.
2. Description of the Related Art
With a non-ringing terminal in the prior art that is connected to a pair of telephone lines, non-ringing call with a telephone station is implemented after identifying whether an incoming call is a normal call or a non-ringing call. Since non-ringing call is implemented by utilizing idle time on a telephone line when there are no normal calls, it is necessary to distinguish between the reception of a non-ringing call and the reception of a normal call. The telephone station makes this distinction based upon a change in the polarity inversion speed in a pair of telephone lines and the presence/absence of a bell signal.
In the case of a normal call, the polarity inversion is implemented at high speed from a standby state and then a bell signal with a high voltage amplitude is provided. In the case of non-ringing call, polarity inversion takes place at low speed with no bell signal provided.
A non-ringing terminal in the prior art is provided with a bell signal detection circuit that detects a bell signal by employing a high voltage detection circuit to selectively perform detection over a voltage range that is higher than the voltage between the lines in a standby state and lower than the voltage between the lines that will be used in an insulation test, a circuit that detects polarity inversion and the presence/absence of a bell signal thereafter, a slow polarity inversion detection circuit that detects the reception of a non-ringing call (non-ringing reception) by detecting a slow polarity inversion and the like, in order to differentiate between the reception of a normal call and the reception of a non-ringing call.
However, bell signal detection circuits employing a high voltage detection circuit, circuits for detecting non-ringing receptions, slow polarity inversion detection circuits and the like in the prior art have yet to address the following problems.
Since, in a bell signal detection circuit employing a high voltage detection circuit, the high voltage detection circuit detects an applied voltage that is equal to or greater than a preset voltage, if high voltage noise enters the telephone lines during a standby state, the circuit detects this noise as a bell signal. In addition, there is a problem in that it becomes impossible to implement an insulation test because during such a test, a current runs through the high voltage detection circuit.
While it is conceivable that a circuit capable of preventing high voltage detection during an insulation test could be constituted by employing a constant current circuit in which a constant current runs only within a preset voltage range, its structure would be complicated and, at any rate, if high voltage noise enters the telephone lines during a standby state, such noise would still be detected as a bell signal.
A circuit that detects a non-ringing reception by detecting polarity inversion and the presence/absence of a bell signal requires two or more photocouplers and the like that independently output the results of various detection. In addition, there is a problem with a slow polarity inversion detection circuit in that, since it employs both a polarity inversion detector in telephone lines and a holding circuit, its circuit structure becomes complicated and in that it always requires capacitors.